This invention is generally related to television synchronization.
In a television system, the picture is produced on a screen by scanning the video signal in time sequence both in the horizontal direction as lines and the vertical direction as fields. The horizontal lines move progressly in the vertical direction like a sawtooth. The reconstruction of the picture requires that the television signal contains horizontal synchronization signal (sync) to control the timing of the retrace of the horizontal scanning lines, and vertical sync signal to control the resetting of the horizontal lines to the top of the screen after the scanning lines reach the bottom of the screen to complete one vertical field.
The scanning lines are interlaced in that the horizontal lines of one field are interposed between the scanning lines of the next field. Such interlacing reduces the flickering of the reconstructed picture.
To effect interlacing, the starting position of the first horizontal scan of one field must be offset by half a scan line from that of the next field. To start the two horizontal scan lines at different points along the horizontal direction, equalization pulses are introduced in the synchronization signals. These equalization pulses usually consist of pulses of double the horizontal sync signal frequency and of different pulse width from the horizontal sync pulses as shown in FIG. 1. In this figure, the p, q, r intervals contain the equalization pulses. The s interval contains the video signal, lying between the horizontal sync pulses.
Thus a video signal must include horizontal sync pulses, vertical field sync pulses and equalization pulses. Such a signal is known as a composite sync signal, indicated as period t in FIG. 1.
A conventional television receiver usually has a horizontal oscillator to generate scanning signals and a vertical oscillator to generate field scanning signals. These oscillators usually contain large value capacitors, which are not suitable for integrated circuits.
Ikeda disclosed in U.S. Pat. No. 4,393,405 a method, whereby both the horizontal and vertical scanning signals are derived from a single oscillator. Ikeda obtains the horizontal and vertical scanning signals by dividing a high frequency signal from a voltage controlled oscillator, which is locked in by the horizontal sync signal in a phase locked loop. However Ikeda's system has several drawbacks:
1) Ikeda uses an integrator to obtain a vertical sync signal from the higher frequency horizontal sync signal. The integrator requires long time constant resistor and capacitor which cannot be integrated.
2) Ikeda's system has no provision for detecting the even or odd field, while conventional even/odd field detecting method consists of monostable multivibrator or differentiating circuits. In either case, long time constant resistors and capacitors are required and are impractical to integrate. Without even/odd field detection, interlacing cannot be implemented.
3) Ikeda's system obtains the horizontal and vertical sync signals using only one voltage controlled oscillator in a phase-locked loop to lock in the different sync frequencies with the horizontal sync signal. Since the equalization pulses are present during the vertical synchronization period and have a different frequency and pulse width from the horizontal sync signals, the equalization pulses may prevent the phase-locked loop from locking in or to be immune from noise.
Therefore, the Ikeda's system leaves something to be desired. A need exists to correct the shortcomings of Ikeda's system.